nginz docker image has been flaky recently, and it seems the apk add command doesn't actually run (it says 'OK' but doesn't acutally install these packages; and then the next invocations using curl and gpg fail).

This commit:

* adds apk update (which seems to render the thing less flaky or maybe fixes the problem)
* separates the package installations from the rest and adds a curl -h task to make future failures more obvious of what the problem is